In order to gain a deeper understanding of the afferent mechanisms controlling eye-head movements, we have now begun an investigation of the role of neck afferents. We intend to ascertain whether the execution of centrally programmed head movements is servo-assisted by impulses originating in the neck muscle spindles. The hypothesis that centrally initiated movements are servo-assisted is based on the fact that activity in group IA afferents from the contracting agonist muscles is greater than that from the stretched antagonists (Severin et al., Biophysics 12, 1967; 575). Group IA afferencts are know to facilitate the spinal motoneurons by way of loops involving the spinal cord, the cerebellum and motor cortex. Because during active muscle contraction the alpha motoneurons also receive inhibitory inpur from Golgi tendon organs, it is not clear how these two infputs interact in the course of normal centrally-programmed movements. Tp test this hypthesis of movement servo-assistance, we plan to apply an inertiallad to the head of normal monkeys during randomly selected trials and compare normal and loaded head movements before and after surgical interruption of the afferent input subserving the hypothetical servo-action (section of cervical dorsal roots). We plan to investigate this problem by using a different type of load such as a spring and by recording EMG from neck agonist and antagonist muscles to gain a better understanding of segmntal spinal cord activity. We will also consider whether movement servo-assistance is present in other modes of eye-head coordination, asduring the tracking of a target.